The present invention relates to the general field of turbine engines having one or two optionally ducted fans, and more particularly to controlling the pitch of the fan blades in such turbine engines.
A preferred field of application of the invention lies with turbojets having contrarotating propellers, also known as “open rotors”, comprising two contrarotating propellers, placed downstream (in a “pusher” configuration) or upstream (in a “puller” configuration) of the gas generator. Nevertheless, the invention also applies to turboprops having one or more propulsive propellers.
In a turbojet with one or more propellers, it is known that the pitch (or orientation) of the blades constituting such propellers constitutes one of the parameters enabling the thrust of the turbojet to be managed, in particular by causing the propeller to operate always under the best possible conditions. Specifically, the speed of the propellers is also constant during all stages of flight, and it is the pitch of the propeller blades that serves to vary thrust. Thus, during a stage of cruising flight, it is desired to obtain the lowest possible power on the turbine shaft that is compatible with given traction at a given airplane speed, so as to obtain the best efficiency (i.e. the efficiency that serves to minimize fuel consumption and increase range). Conversely, on takeoff, the highest possible traction is sought in order to cause the airplane to accelerate and then take off.
Varying the pitch of propeller blades requires certain safety measures to be taken in order to ensure that the blades do not remain blocked in certain positions, in particular as a result of a malfunction of the systems for controlling their pitch. For example, under their own centrifugal effect, the blades tend to take up a flat pitch position that corresponds to a pitch in which their chords make an angle of 90° with the axis of rotation of the propeller. However, a blade blocked in this flat position generates little resistive torque and runs the risk of overspeeding, with the resulting danger of losing a blade and/or the rotary hub carrying it. A blade blocked in this position also runs the risk of generating drag that is excessive and unacceptable for the airplane. Likewise, in flight, moving into a reverse thrust mode (corresponding to a pitch for which the chords of the blades form an angle of 120° with the axis of rotation of the propeller) as a result of a failure of the blade pitch control system, presents the risk of making the aircraft uncontrollable and causing it to drop.
It is thus known to seek to limit the angular positions of propeller blades so that: they do not take up a reverse thrust mode or a flat position while in flight; they do not remain in the flat position while the aircraft is on the ground and the engine is running; and they can occupy all allowable positions when the propellers are not rotating. Furthermore, in the event of a failure of the blade pitch control system, the blades should be capable of taking up a “feathered” position, i.e. a pitch in which their chords are aligned with the axis of rotation of the propeller, thereby reducing the drag that they generate and generating sufficient resistive torque to prevent overspeeding.
In the context of the present invention, these constraints on limiting the angular positions of and on feathering the blades of propellers need to be incorporated in a blade pitch control system of the type comprising an axial annular actuator situated in a reference frame that is stationary relative to the structures of the engine.
Unfortunately, with that type of control, known devices for locking pitch and feathering propeller blades present numerous drawbacks. By way of example, mention may be made of heavy systems using counterweights that generate centrifugal force enabling blades to be feathered, as described in publications WO 2012/066240 and FR 2 957 329.